1. Field of the Invention
The invention relates to current sources and more particularly to controllable large current sources supplying large current to high power electronic devices.
2. Description of the Related Art
FIG. 1 shows a conventional current source 100, which generates a constant supply current Io. The conventional current source 100 comprises a TLV431 regulator 102 and a transistor 104 operating as a current generator. The TLV431 regulator 102 maintains the voltage value of node 106 at a first voltage level Vref. A constant current I1 flows through a first resistor R1, wherein I1=Vref/R1. In transistor 104, the emitter current approximates the collector current. A reference input terminal of the TLV431 regulator 102 is coupled to node 106 and has high input impedance. The supply current Io, therefore, approximates the constant current I1(Io≈I1). If the supply current Io deviates from the constant current I1 (wherein I1=Vref/R1), the voltage level of node 106 deviates correspondingly from the first voltage level Vref. The voltage deviation is detected and adjusted by the TLV431 regulator 102. As shown in FIG. 1, node 110 is connected to a control terminal of the current generator implemented by the transistor 104. The control terminal is the base terminal of the transistor 104. The supply current Io is determined by the voltage value of the control terminal 110. Along with the variation in the supply current Io, the voltage value of node 106 can be adjustable. The TLV431 regulator 102 adjusts the voltage level of the control terminal 110 and adjusts the base current of the transistor 104 to control the supply current Io to maintain the voltage value of node 106 at the first voltage value Vref, and therefore the supply current Io can maintain at the constant value Vref/R1.
With the conventional current source 100, however, the supply current Io is insufficient for high power electronic devices. For example, when the first voltage level (Vref) of the TLV431 regulator 102 is 1.24V, the first resistor R1 is set to 1.24 Ohm to generate a supply current Io of 1 Amp for a load 108. The power consumption of the first resistor R1 is 1.24 W (evaluated from P=I·V=1 A·1.24V=1.24 W). Currently, 1.24 W is considerably large for a chip. In general, the conventional current source 100 is designed to generate a supply current less than 500 mA. A current source generates large supply current for high power electronic devices such as direct current motors, power LEDs, or energy generators and others is thus called for.